Chemistry-potassium permanganate lab

Q2: A bottle containing a standard solution of potassium permanganateis found to have brown stains on the inside. Why will this potassium permanganate be of no further use for quantitative experiments?

KNOWN:
Potassium Permanganate = KMnO4
It is known that potassium permanganate is a common lab oxidizing agent [it reduces]. It naturally has an intense purple color but when potassium permanganate is reduced all the way to Mn2+, it becomes colourless. A solution is titrated when a faint purple colour remains.1/2 Reactions:
The reduction of permanganate:
In acid: MnO4- + 8H+ + 5e- ==> Mn2+ + 4H2O
In neutral solution: MnO4- + 4H+ + 3e- ==> Mn2+ + 2H2O
In base: MnO4- + e- ==> MnO42+

See that those show the oxidizing power seems higher in acid conditions than in basic conditions. This might indicate a quantitative advantage but I'm not really sure how to justify this (maybe someone familiar with the nernst equation knows how). The manganese ion probably does not sit in solution just as the ion but may form an oxide brown precipitate; which condition necessary, not sure.

......... also, jessica.so, you should use either more careful text use or apply the proper tag to express certain information.
MnO42+ is NOT what you mean. You mean MnO4+2 (that is if we can stick two tagged expressions together). (but somehow, that can not be what you meant, either. How do you reduce an ion and have a result with a higher positive charge?).